In Dense Wavelength Division Multiplexed (DWDM) optical communication systems an optical signal, having a total optical power, is actually a composite optical signal made up of multiple carrier wavelengths. Each wavelength is typically used to define an optical channel on which information can be transported. Thus, at any given instant the total optical power of a DWDM optical signal is the summation of the optical power over all the wavelengths that make up the DWDM optical signal.
Optical amplifiers are used to boost the power of signals travelling along an optical fiber. When operated under fixed pump current drive conditions, an optical amplifier output power is constant and for the most part independent of input power levels. For this reason, when amplifiers that carry a number of wavelengths have wavelengths dropped at their input, surviving channels see an increase in power due to the power transferred from the dropped wavelengths. Similarly when wavelengths are added at the input, the total power that was originally distributed over the wavelengths present beforehand, must be redistributed across a larger number of wavelengths, causing the originally present wavelengths to undergo a reduction in power. Power reductions of that nature can cause bit errors to occur at receivers, while power increases can cause not only bit errors but also physical damage to receivers. Power changes due to wavelengths being added or dropped are typically referred to as transients.
By effectively controlling the gain of an optical amplifier, transients can be effectively controlled when wavelengths are added and dropped. A gain control mechanism for an optical amplifier is commonly referred to as an Automatic Gain Controller (AGC).
An AGC in combination with an optical amplifier creates a constant total power gain or ratio of total output power over total input power, regardless of the input power level and wavelength composition of the optical amplifier input. The pump current drive conditions of the optical amplifier are controlled by the AGC to achieve a constant total power gain target.
An optical amplifier alone or in combination with an AGC does not typically account for the fact that a DWDM optical signal is made up of a number of constituent optical wavelengths. A DWDM optical signal input to the optical amplifier is simply treated as a whole having a total optical power that will be boosted accordingly. That is, in some instances when an optical amplifier is operated under fixed pump current drive conditions, all of the powers for the individual outputted wavelengths sum to a fixed total power level regardless of the number of wavelengths. In the event of a switching event, fiber disconnect or equipment failure, where one or more wavelengths is added to or dropped from an optical amplifier input, the AGC must adjust the pump current value(s) such that the ratio of total output power over total input power is substantially constant.
However, the pump current drive adjustment from the AGC is typically much slower than the time taken for the surviving channels to experience a power change. The speed of an AGC is typically limited by stability problems since an AGC must be slow enough to maintain stability over a wide range of operating conditions.
Consequently, the wavelengths then outputted by the optical amplifier (after an abrupt change in the input DWDM optical signal) experience fast transient power excursions that can be harmful to the equipment of the optical system and data carried on it. Specifically, the powers excursions can cause high bit error rates (BER's) when wavelengths are abruptly added to the input stream and damage to receivers and/or high BER's when wavelengths are abruptly dropped from the input stream. The transients, although large in magnitude, are extremely fast (having time constants in the range of tens of microseconds to milliseconds); thus, they are difficult for conventional AGC implementations to control. The net effect has been that while wavelengths outputted from optical amplifier will eventually be forced back to their initial (target) power level in steady state by pump current drive adjustment from the AGC, they will undergo significant power excursions during abrupt switching events.
Exotic AGC designs incorporate a feed-forward path that influences the pump current drive provided by the AGC based on changes in the input power. While this greatly improves the AGC speed, low frequency data content in the measured input power can cause false triggering of the AGC, leading to undesired controller behaviour (e.g. instability and error injection) during steady state conditions when the pump current drive provided by the AGC output should remain fixed. That is, the AGC control (pump current drive) current fluctuates erratically in response to minor fluctuations of the input wavelength powers resulting in a deteriorated level of control over the total optical output power level of the optical amplifier.